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Discussion of precise time and frequency measurement

GPS message jitter (was GPS for Nixie Clock)

Mark Sims

Tue, Jul 19, 2016 9:29 PM

A lot of GPS receivers either don't support the ZDA message, or turn it off by default. And many (most?) that do send it send the time with millisecond resolution.

The NMEA sentences for sending date and time were very poorly thought out. Several different sentences can contain the time (maybe the same time in different sentences with a group, maybe each sentence has a different time). Only the ZDA and RMC sentences have the date. There is no sentence with unified time/date, position, and velocity info.

So for a GPS or a timing unit on a NMEA-0183 bus, do they report their best

estimate of time to the nearest 10ms when the packet is sent? (since a PPS
line isn't shared)

A lot of GPS receivers either don't support the ZDA message, or turn it off by default. And many (most?) that do send it send the time with millisecond resolution. The NMEA sentences for sending date and time were very poorly thought out. Several different sentences can contain the time (maybe the same time in different sentences with a group, maybe each sentence has a different time). Only the ZDA and RMC sentences have the date. There is no sentence with unified time/date, position, and velocity info. ----------------------- > So for a GPS or a timing unit on a NMEA-0183 bus, do they report their best estimate of time to the nearest 10ms when the packet is sent? (since a PPS line isn't shared)

Gary E. Miller

Tue, Jul 19, 2016 10:32 PM

Yo Mark!

On Tue, 19 Jul 2016 21:29:24 +0000
Mark Sims holrum@hotmail.com wrote:

The NMEA sentences for sending date and time were very poorly thought
out. Several different sentences can contain the time (maybe the
same time in different sentences with a group, maybe each sentence
has a different time). Only the ZDA and RMC sentences have the
date. There is no sentence with unified time/date, position, and
velocity info.

Yup, when you are talking NMEA. There are a number of proprietary
binary messages that do. Most of them are badly thought out in
different ways.

In practice, there is often no advantage to the binary over the NMEA
when gpsd is done with it.

RGDS
GARY

Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
gem@rellim.com Tel:+1 541 382 8588

Yo Mark! On Tue, 19 Jul 2016 21:29:24 +0000 Mark Sims <holrum@hotmail.com> wrote: > The NMEA sentences for sending date and time were very poorly thought > out. Several different sentences can contain the time (maybe the > same time in different sentences with a group, maybe each sentence > has a different time). Only the ZDA and RMC sentences have the > date. There is no sentence with unified time/date, position, and > velocity info. Yup, when you are talking NMEA. There are a number of proprietary binary messages that do. Most of them are badly thought out in different ways. In practice, there is often no advantage to the binary over the NMEA when gpsd is done with it. RGDS GARY --------------------------------------------------------------------------- Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703 gem@rellim.com Tel:+1 541 382 8588